The invention relates to a knee prosthesis.
Knee prostheses comprising a femoral component, a tibial component and a meniscal component are well known. Originally, all total knee prostheses had a fixed meniscal component. The concept of a mobile meniscal component is relatively new and is currently gaining considerable acceptance in the market. There is clear evidence that a mobile meniscus does have patient benefits. A mobile meniscus will be more appropriate for a patient with good knee ligamenture, rather than a patient who has a substantially osteo-arthritic deformed knee. There is, therefore, benefit n being able to choose between a fixed meniscus and a mobile meniscus inter-operatively and there is also benefit in having a knee system xe2x80x9con the shelfxe2x80x9d which can be used in either form.
The selection of the prosthesis depends naturally, on the surgeon assessing the patient""s requirements correctly. This can be difficult and the surgeon may, on occasion, be forced to make assumptions which eventually turn out to be incorrect. However, once a prosthesis has been implanted, it is extremely disruptive and inconvenient to remove it and replace it with a more appropriate prosthesis. For example, a surgeon may implant a prosthesis which allows limited relative movement between the tibial and meniscal components only to find at a later date that the patient does in fact require a prosthesis which allows no relative movement. In order to rectify the situation, the prosthesis may have to be removed completely and replaced.
GB-A-2304051 describes a knee prosthesis which allows the manner of operation of the knee to be varied to suit the patient""s needs once the tibial and femoral components are in place by appropriate selection of the meniscal component. For example, one selectable meniscal component may be configured so as to be able to move in a limited manner in any one of the anterior-posterior, medial-lateral and rotational directions or in any combinations of these. Another meniscal component may be configured so as to engage with the tibial component such that no movement relative thereto is permitted. Thereby, the surgeon can select the manner of operation of the knee prosthesis from a number of possibilities without needing to select, order, obtain and check A-complete knee prosthesis.
A further advantage of this known prosthesis is that, should the implanted knee prove inadequate in its manner of operation, it is a relatively simple matter to replace the existing menisc component with an alternative which may perform better.
In this previously known prosthesis, the tibial component has two upstanding bollards which co-operate with recesses in the meniscal component. One of these bollards is slightly under-cut so that it can be brought into snap fitting engagement with a stepped recess in the meniscal component configured so as to engage with a tibial component such that no movement relative thereto is permitted. However, the other bollard is not under-cut and does not snap fit into the corresponding recess of the aforementioned meniscal component.
According to the present invention, there is provided a knee prosthesis comprising, in combination, a tibial component, a femoral component and two meniscal components, one of the meniscal components being configured to co-operate with the tibial component such that relative movement therebetween is prevented and the other meniscal component being configured to co-operate with the tibial component such that limited relative movement therebetween is allowed, the tibial component having two fixed bollards projecting upwardly from an upper surface thereof, the two bollards having annular outwardly extending portions engageable in corresponding recesses in the said one meniscal component in a snap-fit manner.
The fixed meniscal component of such a knee prosthesis has greater anti-destraction strength than is the case if only one bollard snap tits into a corresponding recess of the meniscal component.
Preferably, said other meniscal component has two recesses for receiving the two bollards, respectively, in such a way that limited relative movement between said other meniscal component and said tibial component is allowed, the walls of the recesses in said other meniscal component being provided with grooves to receive the annular outwardly projecting portions of the two bollards. The meniscal components are typically formed of ultra-high molecular weight polyethylene which tends to abrade to form polyethylene debris which may cause osteolysis and resultant failure of the knee prosthesis. The aforesaid grooves have the advantage that they prevent annular outwardly projecting portions of the two bollards from creating polyethylene debris. They also have the further advantage that they discourage vertical separation of the tibial and meniscal components.
Advantageously, one of the recesses of said one meniscal component has a ramp surface so as to allow its respective bollard to slide into place in said one recess. This allows the meniscal component to be glided into place between the tibial and femoral components during the course of an operation.
Preferably, the other recess of said one meniscal component is configured so that its respective bollard latches therein both horizontally and vertically.